Among conventional work vehicles, there exists a wheel loader equipped with an engine, a hydraulic pump coupled mechanically to the output shaft of the engine, hydraulic actuators operated with a fluid supplied from the hydraulic pump, and an automatic transmission operating on a torque converter coupled mechanically to the output shaft of the engine (the wheel loader will be called the torque converter-equipped vehicle hereunder). The torque converter-equipped vehicle travels by having the engine power transmitted to its wheels through the automatic transmission, propeller shafts, differential gears, and drive shafts. Also, the torque converter-equipped vehicle has its front mechanism operated by extending and contracting the hydraulic actuators to move a bucket to excavate, load, and haul gravel or the like.
When the torque converter-equipped vehicle travels, the consumed power of the hydraulic pump (power of the load exerted by the hydraulic pump on the engine) may grow and cause the engine power to become insufficient and the engine revolution speed to drop. In such a case, the torque converter has properties to lower its consumed power (power of the load exerted by the torque converter on the engine) automatically. This allows a balance to be reached between the engine power (supplied power) on the one hand and the total power of the hydraulic pump and torque converter (consumed power) on the other hand at a certain operating point of the engine. At this point, the consumed power of the torque converter drops and so does the power for traveling. However, in a transient state where the engine revolution speed is varied, the rotational energy of the engine output shaft works as a buffer that prevents an abrupt drop in traveling power.
In recent years, meanwhile, there have been proposed hybrid construction machines each equipped with an engine, a hydraulic pump coupled mechanically to the output shaft of the engine, hydraulic actuators operated with a fluid supplied from the hydraulic pump, a motor generator coupled mechanically to the engine output shaft, an electrical motor operated by electrical power supplied from the motor generator, and an electrical storage device that exchanges electrical power with the motor generator and electrical motor. This type of hybrid construction machine is constituted as a two-input, two-output system that allows the hydraulic pump and electrical motor to consume power supplied from the engine and electrical storage device. For that reason, power control means to control the distribution of supplied power between the engine and the electrical storage device and the distribution of consumed power between the hydraulic pump and the electrical motor is a critical element in determining fuel efficiency and operational performance.
JP-2007-247230-A discloses a hybrid construction machine which, in order to prevent a deficiency of power, detects the consumed motive power of a hydraulic pump and that of an electrical motor (called the consumed power hereunder) to control the power of the hydraulic pump and electrical motor in such a manner that the total sum of the consumed power detected will not exceed the total supplied motive power (called the total supplied power hereunder) that can be supplied from the engine and electrical storage device.
Also, JP-2009-216058-A discloses control means aimed at suitably maintaining the operating conditions of the engine, the means setting the rate of increase in engine output power (called the rate of increase hereunder) to a predetermined value to perform control in such a manner that the upper limit of engine output power obtained from the rate of increase will not be exceeded by the demanded power of the hydraulic pump (specifically, the excess in power will be compensated by the output power of the motor generator).